Device for growing single crystals

ABSTRACT

A device for growing single crystals comprising a pair of electromagnetic coils disposed opposed to each other in a symmetry with respect to a central axis of a crucible at an outer side of a heater, a diameter for said coils being greater than 0.8 times of a diameter for said heater and a distance between said coils being greater than 1.5 times of said diameter for said heater.

This invention concerns a device for growing single crystals capable ofimproving the quality of single crystals by applying a magnetic field toa molten material upon pulling up crystals by Czochralski method fromthe molten material which is obtained by heating electroconductivesubstance.

The Czochralski method has been known as a method for producing singlecrystals such as silicon and the method comprises bringing seed crystalsinto contact with the surface of molten liquid of polycrystals melted ina crucible, and then growing single crystals by slowly pulling up theseed crystals under rotation. In this case, the molten materialundergoes thermal convection due to the heat applied from the side ofthe molten material and circulated flow such as centrifugal flow of thesurface layer of the molten material due to the rotation of the seedcrystals. The thermal convection and the circulated flow bring aboutfluctuations of the temperature at the boundary where the singlecrystals grow and, as a result, bring about undesirable effects ofcausing uneven characteristics and defects of crystals to the inside ofthe thus grown single crystals.

In view of the above, in a case if the molten material iselectroconductive substance such as silicon, a parallel magnetic fieldis applied in the horizontal direction to the molten material therebycausing magnetic viscosity to the material and controlling thecirculated flow thereof. As the means for applying the horizontalmagnetic field, there have been used cored electromagnets, or corelesscoils of opposed cylinder type, opposed disc type or opposed saddletype.

However, heating means for the molten liquid is usually comprises anannular carbon member disposed coaxially with the center axis for thecrucible at the outer side thereof and the annular carbon means isalternately disposed with slits from the upper end and slits from thelower end circumferentially, and the electrical heating current flowsthrough the heating means in a zigzag manner along the circumferentialdirection of the heating means. While on the other hand, the electriccurrent is obtained by rectifying an alternative current. Since suchrectified electric current applied to the heating means usually contains3 to 5% of ripples, repeating stresses are applied to the heating meansdue to the effect of a magnetic field crossing in perpendicular to theelectric current flowing in a zig-zag manner along the circumferentialdirection of the heating means and varying the current value thereof toshorten the life of the heating means.

In view of the foregoing problem, the object of this invention is toprovide a device for growing single crystals, in which the ratios of thediameter for the electromagnetic coils and the distance between theelectromagnetic coils relative to the diameter for the cylindricalheating means are respectively within preferred ranges for improving thelife of the heating means for the molten material.

The foregoing object of this invention can be attained by a device forgrowing single crystals comprising a cylindrical crucible, an annularelectric heating means disposed coaxially with a central axis of saidcrucible at an outer side of said crucible for heating and melting anelectroconductive substance in said crucible, said heating means beingadapted such that electric current flows through said heating means in azig-zag manner along a circumferential direction of said heating means,and a pair of electromagnetic coils disposed opposed with each other ina symmetry with respect to said central axis of said crucible at anouter side of said heating means, situated substantially at the sameheight at rotating axes thereof as a liquid surface of said substancemelted in said crucible, a diameter for said coils being greater than0.8 times of a diameter for said heating means and a distance betweensaid coils being greater than 1.5 times of said diameter for saidheating means.

According to the device of this invention, since the ratios of thediameter for electromagnetic coils and the distance between theelectromagnetic coils relative to the diameter for the cylindricalheating means are respectively within preferred ranges, the life of theheating means for the molten material can be extended.

FIG. 1A is a plan view showing the flow of the molten material and themagnetic field in the device according to this invention;

FIG. 1B is a vertical cross sectional view of FIG. 1A;

FIG. 2 is an explanatory view illustrating the relationship between theradius of coils for applying a magnetic field to the molten material andthe gaps between the opposed coils;

FIG. 3 is a graph showing the intensity of the magnetic field on theZ-axis in FIG. 2;

FIG. 4A is a fragmentary vertical cross sectional view for a portion ofthe device according to this invention showing the magnetic fluxesapplied to the molten material;

FIG. 4B is a fragmentary transversal cross sectional view of FIG. 4A;

FIG. 5 is a schematic view showing the limit for the height of the lowtemperature retaining means; and

FIGS. 6A and 6B are explanatory view for the device of growing singlecrystals equipped with an elevatable low temperature retaining means.

FIGS. 1A and 1B are, respectively, a plan view and a vertical crosssectional view illustrating the flow of the molten material and themagnetic field in the device according to this invention, and theprinciple of the method and the device according to this invention willnow be explained referring to these figures.

Since molten material 2 filled in a cylindrical crucible 1 is usuallyheated from the side of the crucible 1, the temperature at the outerside of the molten material 2 is higher than the temperature at thecentral region of the material 2, and convection 3 result to the outercircumference of the molten material 2 as shown in FIGS. 1A and 1B.While on the other hand, the surface layer of the molten material 2 isrotated following after the rotation of silicon single crystals 4, whichcauses circulated flow 5 due to the centrifugal flow of the surfacelayer and the upward flow at the center of the molten material. In thedevice according to this invention, magnetic fluxes 6 are appliedsubstantially along the outer circumference and the bottom of thecrucible 1, by which the magnetic fluxes 6 intersect the convection 3and the circulated flow 5 substantially in perpendicular thereto overthe wide range region of the molten material 2 to effectively suppressthe flow of the molten material 2.

FIG. 2 is an explanatory view illustrating the relationship between theradius of coils for applying the magnetic field to the molten materialand the distance between the opposed coils, in which coils 7 each of anidentical shape and having a radius r are opposed while maintaining adistance l and so as to situate at the center thereof on the Z axis. Inthis case, the intensity of the magnetic field intensity B on the Z-axisis as shown by the graph in the FIG. 3. If r=l, a substantially flatdistribution of magnetic field intensity B is obtained near the center 0for each of the coils 7, while if l>r, the magnetic field intensity B isdecreased near the center 0. While on the other hand, if l<r, themagnetic field intensity B near the center 0 is at the maximum. In thisway, for obtaining a uniform magnetic field intensity B, it isconsidered desirable to make the radius r of the coils and the distancel between the opposed coils as equal as possible. However, it is usuallyset as l>r in view of the limit for the arrangement of the coils.

Explanation will now be made to a preferred embodiment of the device forgrowing single crystals according to this invention.

FIGS. 4A and 4B are, respectively, a fragmentary vertical crosssectional view and a fragmentary transversal cross sectional view for aportion of the device according to this invention illustrating themagnetic fluxes applied to the molten material. A crucible 11 of radiusr made of quartz glass is inserted into a crucible support 10 made ofcarbon and molten material 12 is filled in the crucible. At the bottomof the quartz glass crucible 11, the radius of curvature is r_(b). Whileon the other hand, a grown silicon single crystal body 13 of diameter Dis suspended from a crystal pulling-up wire 14 such that the lowersurface of crystals is in contact with the surface of the moltenmaterial 12. The crucible 11 is contained within a cylindricalelectrical heater 15 of radius r_(h) and the heater 15 is furthercontained in a cylindrical temperature keeping member 16 made of carbon.Flat plate-like super-conductive coils 17 are opposed together withlower temperature keeping means 18 at a distance L on both sides of thetemperature keeping member 16. The super-conductive coreless coils 17are suited to get high magnetic field with large size in view ofeconomy, physical size, weight and electric power consumption. Magneticfluxes 19 from the coils 17 can pass along the outer side of thecrucible 11 and also pass along the bottom of the crucible 11 byproperly selecting the radius r_(c) for the coils 17, the bottom radiusr_(b) for the crucible 11 and the distance L between the coils 17. Thecoils 17 receives a direct electric current.

Referring more specifically to the conditions for the passage of themagnetic fluxes 19 along the outer side and the bottom of the crucible11, it is necessary that the following relationships are establishedbetween the radius of curvature R_(MF) of the magnetic fluxes 19 and theouter diameter r of the crucible 11, as well as between the radius ofcurvature R_(MF) and the bottom radius r_(b) of the crucible 11respectively as:

r≦R_(MF) ≦4r

r_(b) ≦R_(MF) ≦4r_(b).

For satisfying the foregoing conditions, it is desirable that the radiusr_(c) of the coil 17 is in a relationship:

r_(c) =1.5 to 5 times r.

While on the other hand, the heating means for the molten materialusually comprises an annular carbon member disposed coaxial with thecentral axis for the crucible at the outer side thereof and the annularheating means is alternately disposed with slits from the upper end andslits from the lower end circumferentially. Accordingly, electricalheating current flows through the heating means in a zig-zag manneralong the circumferential direction of the heating means. For avoidingthe vibrations of the heater 15 resulted by the interaction between theelectrical current and the magnetic field, it is desirable that theripple value of the heating current of the heater 15 is as small aspossible. However, since heating current usually contains from 3 to 5%ripples for a commercial rectifier used in CZ puller, if the coils 17and the heater 15 are arranged closer to each other in the case wherethere are fluctuations in the current value within the above-mentionedrange, large stresses are exerted repeatingly at a high temperature tothe heater 15 due to the excess magnetic field of the coils 17 and thefluctuating current of the heater 15, accordingly, the working life ofthe heater 15 is shortened. This short working life will be eliminatedby using a highly rectified electric current. But the rectifiersupplying such a highly rectified electric current is of more high priceand large size than a usual rectifier.

As the result of the experiment, if the radius r_(c) of the coil 17 andthe distance L between the coils 17 are selected so as to satisfy:

r_(c) >r_(h), and L>3r_(h),

where r_(h) represents the radius of the heater 15, r_(c) represents theradius of the coil 17 and L represents the distance between the opposedcoils 17, the times capable of using the heater 15 at 4% ripple value ofthe heating current can be increased by from 20 to 30% as compared withthe case:

r_(c) >0.8r_(h) or

r_(c) ≦r_(h), or L≦3r_(h).

By the way, for stably pulling-up the single crystals in a state wherethe magnetic field is applied to the molten material, it is desirablethat there is the relationship: D/2r<0.75 between the diameter D for thesingle crystals 13 and the diameter 2r for the crucible 11 and it hasfurther been found from the result of the experiment that the oxygendensity in the grown single crystals 13 has a concern with the valueD/2r.

That is, in a state where a magnetic field at 3000 gauss is applied tothe molten material 12, the oxygen density of single crystals can bereduced easily to less than 10×10¹⁷ atoms/cm³ if D/2r<0.7 and to lessthan 5×10¹⁷ atoms/cm³ if D/2r<0.6. It is preferable that D/2r<0.5 andthe oxygen density can be as low as 1×10¹⁷ atoms/cm³ in this case.However, the diameter D of the single crystals 13 is greater than 100mmφ in the foregoing experiment.

FIG. 5 is an explanatory view illustrating the limit for the height ofthe low temperature retaining means, in which pulled-up crystals 20 aredisposed within a pulling-up chamber 21. The central line 23 for coils22 substantially coincides with the liquid surface 24 of the moltenmaterial. If the diameter of the coils 22 is increased in the lowtemperature retaining means 25, the outer diameter and the height forthe cylindrical low temperature retaining means 25 having a verticalaxis for opposed flat-plate superconductive coils are increased. In theexperimental device equipped with the cylindrical low temperatureretaining means, it is desirable that the height for the low temperatureretaining means is higher by 300 mm than a height for the upper end ofthe coils, as well as that the inner diameter of the low temperatureretaining means is smaller at least by 100 mm than the distance Lbetween the coils and that the outer diameter of the means is set to avalue greater than: √r_(c) ² +L² /4+100 mm.

Further, in view of the operation of the device, large-scaled lowtemperature retaining means is not favorable and it is desirably limitedto such a height that a measuring wind 26 functions satisfactorily. Thatis, the outer size of measuring means 27 for optically measuring thediameter of the single crystals 20 is about 10 cm×30 cm or about 7cmφ×10 cm. For visually observing the pulled-up single crystals 20, theretaining position for the measuring means 27 has to be situated on aline 28 connecting the center of the boundary of the single crystalswith the center for the measuring wind 26, and the upper limit for theheight is 200 mm above the flange of the pulling-up chamber 20.Accordingly, the limit for the height of the low temperature retainingmeans 25 is less than 200 mm for h in FIG. 5. Within this range, thereare no troubles for the mounting of the low temperature retaining means25 and the operation for the measuring means 27.

It is of course possible to make various modifications for the shape ofthe low temperature retaining means 25. For instance, by using aseparatable type low temperature retaining means, the foregoingrestriction can be moderated. Further, the outer shape of the lowtemperature retaining means may be cylindrical, or may be cylindrical inthe inside and square post-like at the outer side. In this case, oneexample for the dimension and the weight of the low temperatureretaining means for use with a crucible of maximum 450 mm diameter, theinner cylinder diameter was 900 mm, the outer square post size was 1350mm (width)×1460 mm (lateral size)×1135 mm (height), and the total weightwas 2.6 t.

FIG. 6A is an explanatory view for the device of growing single crystalsequipped with an elevatable low temperature retaining means, in which alow temperature retaining means 32 is disposed opposing to pulling-upchamber 31. Upon replacing hot zone constituent parts such as heaters,upon replacing crucibles, upon mounting polycrystalline silicon and uponmaintenance such as cleaning for the inside of the chamber, the lowtemperature retaining means 32 can be downwarded to below the pulling-upchamber 31 to improve the handlability upon maintenance. Further, thepulling-up chamber 31 is supported on a rotational support post 33 and arotational support arm 34, in which the pulling-up chamber 31 is rotatedaround the rotation post 33 to expose the hot zone constituent parts 35thereby improve the operationability upon foregoing maintenance.

What is claimed is:
 1. A device for growing a single crystalcomprising:a circular cylindrical crucible having a bottom of aspherical segment, an annular heating means disposed coaxially with alongitudinal axis of said crucible and surrounding said crucible forheating and melting an electroconductive substance filled in saidcrucible, said heating means being alternatively provided with firstslits from one end thereof and second slits from the other end thereofalong a circumference thereof, means for pulling up said single crystalfrom said molten substance, and a pair of circle electromagnetic coilsfrom applying one-way axially symmetric magnetic field to saidsubstance, disposed outside said heating means and opposed to each otherin a symmetry with respect to said longitudinal axis, situatedsubstantially at the same height at central axes thereof as an uppersurface of said molten substance, wherein a height of said coil isgreater than 0.8 times of a diameter of said heating means and adistance between said coils is greater than 1.5 times of said diameterof said heating means, an effective average radius of said coil beingfrom 1.5 to 5 times of a radius of said crucible, whereby, magneticfluxes of said axially symmetric magnetic field are arranged in such amanner that all other said magnetic fluxes except a central flux passingthrough a rotational axis of said axially symmetric magnetic field areswelled, wherein said rotational axis substantially passes through saidupper surface, said magnetic fluxes include both side lateral fluxeseach passing through a circumferential side portion of said crucible atthe furthest apart portion from said rotational axis and at a level ofsaid upper surface, each of said both side lateral fluxes at said sideportion having a substantially same curvature as that of said sideportion, wherein said magnetic fluxes also include a lower flux passingthrough the lowest portion of said bottom, said lower flux at saidlowest portion having a substantially same curvature as that of saidlowest portion.
 2. A device according to claim 1, in which saidsubstance comprises a polycrystalline silicon.
 3. A device according toclaim 1, in which said coils comprise superconductive coils.
 4. A deviceaccording to claim 2, in which said coils comprise superconductivecoils.